1. Field of the Invention
The present invention relates to a crystal manufacturing apparatus and a crystal manufacturing method in which crystals are grown according to the Czochralski (CZ) method.
2. Description of the Related Art
With recent increases in the degree of integration and precision in semiconductor devices, quality requirements for semiconductor crystal substrates have shown a tendency to become stricter. Semiconductor crystals are primarily grown by the CZ method, and every effort has been made to grow crystals having higher purity, lower defect density, and higher homogeneity. Recently, it has been found that a crystal defect is closely related not only to purity of a raw material, purity of a member used, and accuracy of an apparatus, but also to the thermal history of a crystal during its growth. For example, for silicon, a thermal history has an effect on OSF (Oxidation Induced Stacking Faults), oxygen precipitation, BMD (Bulk Micro-Defect), FPD (Flow Pattern Defect), LSTD (Laser Scattering Tomography Defect), and the oxide dielectric breakdown voltage. Also, for compound semiconductors such as GaP, GaAs, and InP, a thermal history has a considerable effect on dislocation density and a level of such defects as functioning as a donor or an acceptor. Accordingly, there have been proposed crystal manufacturing apparatuses having a variety of furnace structures that control defects in crystals through adjustment of a thermal history during crystal growth (refer to, for example, H. Yamagishi, I. Fusegawa, K. Takano, E. Iino, N. Fujimaki, T. Ohta, and M. Sakurada, Proceedings of the 17th International Symposium on Silicon Materials Science and Technology, SEMICONDUCTOR SILICON 1994, PP.124-135).
However, according to these proposed apparatuses or methods, only the temperature at a certain position within a furnace can be increased or decreased, i.e. the entire temperature distribution within a furnace cannot be adjusted. Further, this control of temperature at a certain position within the furnace is such that the temperature of an entire grown crystal is increased or decreased. That is, this temperature control has no degrees of freedom, and its accuracy is poor. Also, when a temperature distribution needs to be changed to meet a new design requirement, an apparatus must be redesigned from the beginning.
That is, in conventional crystal manufacturing apparatuses or crystal manufacturing methods, the range of control of a temperature distribution within a furnace is very narrow and thus the range of control of the thermal history of a crystal is also very narrow, and in addition the accuracy of the control is very low. Within a single apparatus or furnace structure, it is impossible to change or control a temperature distribution within a furnace and thus to change or control the thermal history of a crystal during growth of a crystal or between crystals. Accordingly, when the thermal history of a crystal must be changed in accordance with a required crystal quality or a standard, an apparatus itself must be changed. In other words, an apparatus must be selected according to a crystal to be grown. This situation narrows the attainable range of quality for crystals and brings about a significant increase in apparatus cost.